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Ytterbium
Ytterbium is an element with the symbol Yb and the atomic number 70. It is a metal, variously described as either silvery or slightly yellowish, that is quite reactive yet stable in air. It is classified as a rare earth metal and a lanthanide, and behaves quite similarly to the other members of this group. Ytterbium is not found as a free metal in nature, but occurs with the other rare earths in minerals such as samarskite, gadolinite, and monazite. Ytterbium has a few specialty applications. It is occassionally used as a dopant in stainless steel to improve workability and corrosion resistance. Infrared lasers may use ytterbium-doped crystals to produce light of a desired frequency. Ytterbium is being investigated as a replacement for magnesium in decoy flares for military aircraft due to the high emission of the metal and its oxide in the infrared range. Specific isotopes are used as gamma ray sources. Properties Physical properties Ytterbium is a silvery metal. Its exact color, however, is disputed: some sources describe it as grayish, others as slightly brassy. It exists in three allotropes: α-ytterbium, which exists in a hexagonal crystal structure is stable below -13 °C and is diamagnetic, β-ytterbium, which is stable at room temperature, paramagnetic and exists in a face-centered cubic structure, and γ-ytterbium, which exists above 795 °C and has a body centered cubic structure. Due to the filled f-shell, the metal has unusually low melting and boiling points, and is also less dense than most other lanthanides. Melting ytterbium (which must be done in an inert atmosphere to prevent ignition of the metal) will invariably cause some to boil off due to the narrow liquid range. This filled f-shell contributes to ytterbium's extremely low magnetism compared to rare earths such as gadolinium and dysprosium. Ytterbium consists of seven stable isotopes. Two of these are expected to be unstable with a half-life longer than the age of the universe. Ytterbium metal changes its resisitivity as pressure or other stresses increase. Chemical properties Ytterbium is a reactive metal, but it does not tarnish quickly in air. It does, however, burn in air to form ytterbium(III) oxide. Ytterbium burns with a characteristic green flame, notably in compositions with hexachloroethane and polytetrafluoroethylene. Ytterbium reacts vigorously with dilute acids to form salts. Most of these salts are soluble in water except for the fluoride, sulfate, and oxalate. All of these salts are white in color. Ytterbium also reacts with the halogens to form trihalides. The metal reacts only slowly in cold water, but vigorously in hot water, to form ytterbium hydroxide, which is basic enough to absorb carbon dioxide from the air. Under reducing conditions, a ytterbium(II) species is known to exist. This ion will reduce water to hydrogen gas and is therefore unstable in aqueous solution. It can be stabilized in tetrahydrofuran. Ytterbium(II) compounds can be formed by reacting metallic ytterbium with ytterbium(III) compounds, but they disproportionate to ytterbium metal and ytterbium(III) compounds at elevated temperatures. Applications Atomic clocks Ytterbium clocks are the most stable known, losing only a second per 10 billion years. Dentistry Alloys of ytterbium have been used in dentistry. Ytterbium(III) fluoride has been used in dental fillings as a source of fluoride. Metallurgy Ytterbium is used as a dopant for stainless steel to improve workability and corrosion resistance. Safety Toxicity Ytterbium compounds have not been investigated for their toxicity, and should be treated as mildly toxic. Ytterbium has no biological role but may stimulate metabolism. Salts of ytterbium tend to hydrolyze at elevated temperatures and may emit noxious or strongly acidic vapors. Flammability Ytterbium as small pieces will ignite in the presence of an open flame, and ytterbium dust and powder may ignite very easily. Class D fire extingushers should be at hand. Water may aggravate burning ytterbium and should never be used.